1. Field
This patent relates generally to devices and methods for measuring fluid properties for oilfield and other industrial applications. In particular, the patent relates to measuring one or more fluid properties such as density.
2. Background
The ability to measure fluid density and other fluid properties downhole is paramount to petroleum exploration as it enables one to differentiate between oil, gas, and water. W. D. McCain, Jr., The Properties of Petroleum Fluids, 2nd ed. (1990). The relative amounts of oil and gas produced have a direct impact on reservoir development cost. Furthermore, it allows one to locate the oil-water contact line and hence the thickness of the pay zone of a formation. This ability hinges upon the availability of an accurate and robust sensor that works reliably in the harsh environment found in an oil well. Oilfield pressures downhole may be as high as 25,000 psi with temperatures up to 175° C. or higher. There are wells with even more extreme conditions, especially offshore. A further challenge in downhole fluid analysis is that acquiring large quantities of representative downhole fluids is difficult due to ever-present contamination, such as from drilling mud or formation water. O. C. Mullins, M. Hashem, H. Elshahawi, G. Fujisawa, C. Dong, S. Betancourt, T. Terabayashi, Petrophysics 46, 302 (2005).
Fluid density provides a means of fluid typing. Water has a density of 1.0 g/cm3. Densities of liquid-rich hydrocarbons vary between 0.5 to 0.9 g/cm3, and dry gas or condensate formations have a significantly lower density. The above values are understood to vary with reservoir pressure and temperature. Further, an understanding of the heterogeneity of the reservoir may require that densities be measured at several depths so that the compositional variation can be fully deduced. Such information can aid in identifying zones with the highest economic value; for example a dry gas may be preferred if the well is drilled in a gas field with existing infrastructure to handle gas transport through pipelines. Furthermore, for a given gas composition, the amount of standard cubic feet (SCF's) of gas producible is directly proportional to the density. Knowledge of the fluid density is essential for avoiding costly economic errors in applications such as, by non-limiting example, oilfield applications.
It is straight-forward to measure a liquid density at ambient pressure and temperature in a laboratory setting. Typically a flask of well-defined volume (volumetric flask) is filled with the fluid of interest and it is weighed on a scale. The density is obtained by dividing the fluidic mass by the known volume. Measurements at elevated pressure and temperature, however, require more sophisticated techniques. Gaseous fluids and heterogeneous liquids further complicate the measurement task. There are some known measurement methods employed for pressure/temperature density measurements, but they have limitations for downhole implementation.
For example, a known method includes using a resonating sensor such as a resonating tube densitometer and a Coriolis flow meter. However, such methods are not well-suited for subterranean applications. Firstly, the sizes of such flow meters in many cases are too large to fit in downhole tools such as, by non-limiting example, oilfield applications. Secondly, mud can be omnipresent at the beginning of a job (i.e., an oilfield application job), and it is unclear whether that mud can be completely cleaned from the two flow lines of the sensor. Thirdly, the method operates at pressures in the order of 1,000 psi, which makes such a method unusable at pressures much higher than 1,000 psi.
Furthermore, there are special fluid conditions that can present problems for resonating sensors in measuring fluid properties, such as density. For example, some of the problems with resonating sensors include inaccurate measurements of fluid properties, such as density, due to special fluid conditions, i.e., gaseous fluids, emulsions, non-Newtonian fluids, supercritical fluids or multiphase fluids.
Therefore, there is a need for a device, method, and system that can measure fluid properties such as density for oilfield applications and other industries that can overcome the above noted problems either above ground or in a subterranean environment.